An electrical rotating machine is typically an electric motor or a power generator made up of a stator, a rotor, and a housing supporting the rotor and the stator. The electrical rotating machine can be classified into a normal conducting electrical rotating machine using normal conducting coils, which do not cause a superconducting phenomenon, and a superconducting electrical rotating machine using superconducting coils, which cause the superconducting phenomenon. The superconducting electrical rotating machine has a so-called radial gap type structure in which mainly, a cylindrical stator and a plurality of field poles (superconducting field poles) are arranged. In the stator, an armature winding of a plurality of phases using a normal conducting winding is arranged in a circumferential direction, and the plurality of field poles using superconducting coils are disposed coaxially with the stator in an internal space of the stator, and are arranged in the circumferential direction so as to be opposed to the plurality of phases of the armature winding of the stator. In the rotor of the superconducting electrical rotating machine, a rotor core as an inner cylindrical body and a casing as an outer cylindrical body surrounding an outer circumference of the rotor core are rotatably supported by a rotor shaft joined on a central axis of the rotor core. Moreover, the rotor of the superconducting electrical rotating machine is formed with a decompression space between the rotor core and the casing, and in this decompression space, the superconducting field poles are arranged.
As shown in FIG. 14, a superconducting field pole 28 has a structure in which a plurality of racetrack type coils 29 are laminated, the racetrack type coils 29 being each made up of a pair of linear portions 30a, 30b opposed to each other, and a pair of arc portions 30c, 30d opposed to each other and joining both ends of the linear portions 30a, 30b. Specifically, the superconducting field pole 28 shown in FIG. 14 is a superconducting coil body in which four racetrack type coils 29a to 29d are laminated, and the racetrack type coils 29a to 29d of respective layers each have an air-cored structure, which does not configure a so-called magnetic circuit, obtained by spirally (like mosquito repelling incense) winding a tape-like (belt-like) superconducting wire material 31 like a pancake around an oval winding frame not shown so as to form a racetrack shape. Further, a cross section of each of the racetrack type coils 29a to 29d shown in FIG. 14 has a double-layered structure accompanying double pancake winding. That is, the two layers of the superconducting wire material 31 are formed in a lamination direction of the racetrack type coils 29 so that a longitudinal direction (a parallel direction) of a cross section of the superconducting wire material 31 is the lamination direction of the racetrack type coils 29, and a short direction (a vertical direction) of the cross section of the superconducting wire material 31 is a radial direction of the racetrack type coils 29. In addition, the cross section has a shape in which the relevant two layers of the superconducting wire material 31 are arrayed from a radially inner side to a radially outer side of each of the racetrack type coils 29. The racetrack type coils 29 may each have a single-layered structure by single pancake winding besides the double-layered structure by the double pancake winding.
In the above-described structure of the superconducting field pole 28, a critical current, which is one of performance measures of the superconducting wire material 31, depends on intensity of a magnetic field (hereinafter, referred to as a vertical magnetic field) acting in a direction vertical to a tape broad width face (main face) of the superconducting wire material 31 (in the radial direction of the racetrack type coil), so that there has been known a problem that as the intensity of the vertical magnetic field becomes larger, the critical current is reduced. FIG. 15 is a schematic view indicating a situation where the vertical magnetic field occurs.
On the other hand, in Patent Literature 1, (PTL1), Japanese Unexamined Patent Application Publication No. H7-142245, there has been disclosed a high-temperature superconducting magnet to which flanges each made of iron such as, for example, a silicon steel sheet and the like as ferromagnetic bodies are attached at both ends of a high-temperature superconducting coil body in which a plurality of high-temperature superconducting coil units using a high-temperature superconducting tape material are laminated. In this manner, when the ferromagnetic bodies are attached at both the ends of the high-temperature superconducting coil body, a magnetic field of a coil winding portion is directed to the ferromagnetic bodies. As a result, it is said that as compared with a high-temperature superconducting magnet to which no ferromagnetic body is attached, in the high-temperature superconducting magnet to which the ferromagnetic bodies are attached, reduction of a critical current density by the magnetic field of the high-temperature superconducting tape material is small, and a generated magnetic field of the high-temperature superconducting magnet is increased.